Aircraft frequently encounter stresses during operation. For example, airflow may tend to twist or bend the wings even under standard conditions. To overcome the potential dangers of operational stresses, most aircraft designs include systems for maintaining structural integrity.
Two alternatives in aircraft design are monocoque designs, in which the shell material provides a substantial degree of structural support, and frame designs, in which an internal framework bears a substantial portion of the load. Whether a monocoque design is more suitable than a frame design in a specific instance may depend on a variety of factors, including the cost of materials and the expected operating conditions. A hybrid approach involving both internal structure and load bearing skin may be employed in situations where optimization favors neither extreme.
Regardless of the design, the wings and fuselage generally comprise separate components that are attached during the manufacturing process. Inasmuch as lift is generated within the wings rather than within the fuselage, this assembly step is usually critical to ensure reliability of the aircraft. Accordingly, this step usually involves permanent attachment of the wings to the fuselage as by fasteners, welding, and adhesives.
While permanent attachment may provide a reliable connection between the wings and fuselage, it may not be optimal in some circumstances. For example, storage of aircraft is generally not volumetrically efficient when the wings remain extended with respect to the fuselage. To overcome these problems, solutions such as folding of the wings, as in the Vought F4U Corsair and Grumman TBF Avenger, have been proposed. However, many of these solutions add complexity and may reduce operational reliability. In addition, many aircraft require low costs, light weight, and low part counts, along with performance demands such as high payload weight capacities, large payload volumes, and good flying characteristics.